Engine

ABSTRACT

There is provided a method for converting a turbofan engine, including the step of providing a turbofan engine and the step of converting the turbofan engine to provide a converted turbofan engine. The turbofan engine includes a core engine (including at least one high pressure spool assembly and a combustion chamber), and an unmodified fan configured for providing at least a bypass flow bypassing the core engine, the fan being mechanically coupled to a low pressure turbine that is in turn driven by the core engine. The conversion step includes modifying or replacing the unmodified fan to provide a modified fan, the modified fan being configured for generating a reduced bypass flow with respect to said fan bypass flow during operation of the converted turbofan engine corresponding to at least one set of engine conditions corresponding to step (a), thereby enabling said low pressure turbine to generate an excess shaft power above a baseline shaft power required for driving the modified fan during operation of the converted turbofan engine. Also provided is a corresponding converted turbofan engine.

TECHNOLOGICAL FIELD

The presently disclosed subject matter relates to aeroderivative gasturbine engines, particularly for the generation of electrical power.

BACKGROUND

Gas turbines have been used for many years for the generation ofelectrical power. Such gas turbines, often referred to as land-based gasturbines, can be divided into two general groups: industrial engines andaeroderivative engines.

Industrial engines are purpose-built for electrical generation or otherland-based uses, while aeroderivative engines are derived fromaeronautical gas turbine engines that are originally designed foraeronautical applications such as propulsion units for aircraft, forexample. While industrial engines tend to be physically large and heavy,aeroderivative engines are generally far more compact.

Some types of aeroderivative engines are provided from gas turbineengines that have been previously used for aeronautical applications. Insuch cases, the gas turbine engines are refurbished, and converted forelectrical power generation.

The conventional approach for converting a turbojet type gas turbine toprovide an aeroderivative gas turbine is to replace the exhaust nozzlewith an aerodynamically coupled power turbine, which generates shaftpower which in turn can be used to drive an electrical power generator,such as an alternator for example, to generate electrical power.

The conventional approach for converting a turboprop type gas turbine toprovide an aeroderivative gas turbine for electrical power generation,for example, is to remove the propeller and gearbox from the powerturbine, and to mechanically couple the power turbine of the turbopropto an electrical power generator.

The conventional approach for converting a turbofan type gas turbine toprovide an aeroderivative gas turbine is to convert the turbofan engineto a turbojet engine, by replacing the fan and bypass system with anadditional low pressure ratio compressor, and by replacing the exhaustnozzle with an aerodynamically coupled power turbine, which generatesshaft power which in turn can be used to drive an electrical powergenerator.

GENERAL DESCRIPTION

According to an aspect of the presently disclosed subject matter, thereis provided a method for converting a turbofan engine, comprising;

-   -   (a) providing a turbofan engine, the turbofan engine comprising;        -   a core engine including at least one high pressure spool            assembly and a combustion chamber;        -   an unmodified fan configured for providing at least a bypass            flow bypassing the core engine at least at one set of engine            conditions, the fan being mechanically coupled to a low            pressure turbine that is in turn driven by the core engine;    -   (b) providing a converted turbofan engine from the turbofan        engine by modifying or replacing the unmodified fan to provide a        modified fan, the modified fan being configured for generating a        reduced bypass flow with respect to said fan bypass flow during        operation of the converted turbofan engine corresponding to said        at least one set of engine conditions of step (a), thereby        enabling said low pressure turbine to generate an excess shaft        power above a baseline shaft power required for driving said        modified fan during operation of the converted turbofan engine.

Thus, in operation of the converted turbofan engine said low pressureturbine generates the aforesaid excess shaft power, wherein the excessshaft power is in excess of the baseline shaft power that is requiredfor driving said modified fan during operation of the converted turbofanengine. The excess shaft power can be used for a variety of uses, forexample generation of electrical power.

For example, in step (b) said modified fan is modified with respect tothe unmodified fan by reducing the outer diameter of the fan blades ofsaid unmodified fan. Additionally or alternatively, in step (b), saidmodified fan is modified with respect to the unmodified fan by removingat least an outer radial portion of the fan blades of said unmodifiedfan. Additionally or alternatively, in step (b), said modified fan ismodified with respect to the unmodified fan by removing the fan bladesof said unmodified fan.

Alternatively, for example, in step (b), said modified fan is modifiedwith respect to the unmodified fan by modifying the geometry of at leastan outer radial portion of the fan blades of said unmodified fan suchthat the said at least outer radial portion of the fan blades generatesreduced bypass flow as compared with the unmodified fan blades.Optionally, said at least outer radial portion of the fan blades ismodified by providing a zero or near zero angle of attack with respectto a direction of an airflow into the converted turbofan engine.

Additionally or alternatively, in step (a), the unmodified fan is alsoconfigured for providing a core flow through the core engine.Optionally, said unmodified fan is further configured for providing afirst pressure ratio to a core flow to the core engine, and wherein saidmodifying or replacing the unmodified fan to provide a modified fan issuch as to provide instead a second pressure ratio to the core flow tothe core engine replacing said first pressure ratio, wherein said firstpressure ratio is similar in magnitude to said second pressure ratio.Alternatively, said unmodified fan is further configured for providing afirst pressure ratio to a core flow to the core engine, and wherein saidmodifying or replacing the unmodified fan to provide a modified fan issuch as to provide instead a second pressure ratio to the core flow tothe core engine replacing said first pressure ratio, wherein said firstpressure ratio is smaller in magnitude to said second pressure ratio.Alternatively, said unmodified fan is further configured for providing afirst pressure ratio to a core flow to the core engine, and wherein saidmodifying or replacing the unmodified fan to provide a modified fan issuch as to provide instead a second pressure ratio to the core flow tothe core engine replacing said first pressure ratio, wherein said firstpressure ratio is larger in magnitude to said second pressure ratio.Alternatively, said unmodified fan is further configured for providing afirst pressure ratio to a core flow to the core engine, and wherein saidmodifying or replacing the unmodified fan to provide a modified fan issuch as to provide instead a second pressure ratio to the core flow tothe core engine, wherein said second pressure ratio is 1.0.

Additionally or alternatively, in step (b), said modified fan isconfigured to produce a reduced bypass thrust, as compared with thebypass thrust generated the unmodified fan.

For example, additionally or alternatively to the above, said modifiedfan and said low pressure turbine are provided in a low pressure spoolassembly.

For example, additionally or alternatively to the above, the methodfurther comprises operatively coupling the converted turbofan engine toan electrical generator to enable conversion of said excess shaft powerto electrical power. Additionally or alternatively, the method furthercomprises operatively coupling the converted turbofan engine to amechanical load to apply said excess shaft power to the mechanical load

For example, additionally or alternatively to the above, said turbofanengine in step (a) is a multi-spool, high bypass, forward fan, turbofangas turbine engine, wherein said unmodified fan is forward mounted. Forexample, the converted turbofan engine is selectively coupled to theelectrical generator via the modified fan; optionally, the convertedturbofan engine is further selectively coupled to an additionalelectrical generator via the low pressure turbine. For example, theconverted turbofan engine is coupled to the electrical generator via thelow pressure turbine.

For example, additionally or alternatively to the above, said turbofanengine in step (a) is a multi-spool, high bypass, aft fan, turbofan gasturbine engine, wherein said unmodified fan is aft-mounted. For example,in step (a), the unmodified fan is configured for providing only abypass airflow, i.e., no core flow is provided by the unmodified fan.Additionally or alternatively, said modified fan and said low pressureturbine are provided in a single rotor assembly. Additionally oralternatively, the converted turbofan engine is coupled to theelectrical generator via the low pressure turbine. Additionally oralternatively, said reduced bypass thrust is nominally zero.

Additionally or alternatively, the method further comprises providing athrust bearing arrangement for the low pressure turbine for balancing anext axial force corresponding to said excess shaft power.

Additionally or alternatively, the method further comprises maintainingthe engine pressure ratio of the turbofan engine in the modifiedturbofan engine. For example, maintaining the engine pressure ratio ofthe turbofan engine in the modified turbofan engine comprises modifyingthe low pressure turbine in the modified turbofan engine to provide aflow cross-sectional flow area through the low pressure turbine at anaxial location thereof that is larger than the corresponding flowcross-sectional flow area in the turbofan engine.

Additionally or alternatively, the method further comprises replacing acore nozzle of the unmodified engine of step (a) with an exhaustdiffuser to the core engine in step (b), wherein the exhaust diffuser isconfigured for reducing a core engine thrust as compared to a corethrust of the core engine in the unmodified turbofan engine.

For example, additionally or alternatively to the above, said at leastat one set of engine conditions includes at least one of the designpoint of the turbofan engine and the maximum continuous cruise at ISAconditions of the turbofan engine.

According to an aspect of the presently disclosed subject matter, thereis also provided a turbofan engine, comprising;

-   -   a core engine including at least one high pressure spool        assembly and a combustion chamber;    -   a modified fan mechanically coupled to a low pressure turbine        that is in turn driven by hot gases generated by the core        engine;    -   wherein said modified fan is derived from or replaces an        unmodified fan;    -   wherein said low pressure turbine is initially designed to drive        the unmodified fan, when coupled thereto in place of the        modified fan, the unmodified fan being configured for providing        at least a bypass flow bypassing the core engine at least at one        set of engine conditions when driven by said low pressure        turbine;    -   wherein said modified fan is configured for generating a reduced        bypass flow with respect to said fan bypass flow during        operation of the turbofan engine corresponding to said at least        one set of engine conditions, and for concurrently allowing said        low pressure turbine to generate an excess shaft power above a        baseline power required for driving said modified fan during        operation of the turbofan engine at least at said at least one        set of engine conditions.

For example, said turbofan engine is produced by converting anunmodified turbofan engine, the unmodified turbofan engine comprisingsaid core engine and said unmodified fan, said unmodified fan beingmechanically coupled to the low pressure turbine that is in turn drivenby hot gases generated by the core engine, and wherein said conversionincludes modifying or replacing the unmodified fan to provide themodified fan.

For example, said modified fan is configured to produce a reduced bypassthrust, as compared with the bypass thrust generated by the unmodifiedfan when coupled to the turbofan engine in place of the modified fan.

For example, said modified fan is modified with respect to theunmodified fan by reducing the outer diameter of the fan blades of saidunmodified fan. Optionally, said modified fan is modified with respectto the unmodified fan by removing at least an outer radial portion ofthe fan blades of said unmodified fan. Additionally or alternatively,said modified fan is modified with respect to the unmodified fan byremoving the fan blades of said unmodified fan.

Alternatively, said modified fan is modified with respect to theunmodified fan by modifying the geometry of at least an outer radialportion of the fan blades of said unmodified fan such that the said atleast outer radial portion of the fan blades generates reduced bypassflow as compared with the unmodified fan blades. For example, said atleast outer radial portion of the fan blades is modified by providing azero or near zero angle of attack with respect to a direction of anairflow into the turbofan engine.

For example, additionally or alternatively to the above, the unmodifiedfan is further configured for providing a core flow through the coreengine. Optionally, said unmodified fan is further configured forproviding a first pressure ratio to a core flow to the core engine, andwherein said modifying or replacing the unmodified fan to provide amodified fan is such as to provide instead a second pressure ratio tothe core flow to the core engine replacing said first pressure ratio,wherein said first pressure ratio is similar in magnitude to said secondpressure ratio. Alternatively, said unmodified fan is further configuredfor providing a first pressure ratio to a core flow to the core engine,and wherein said modifying or replacing the unmodified fan to provide amodified fan is such as to provide instead a second pressure ratio tothe core flow to the core engine replacing said first pressure ratio,wherein said first pressure ratio is smaller in magnitude to said secondpressure ratio. Alternatively, said unmodified fan is further configuredfor providing a first pressure ratio to a core flow to the core engine,and wherein said modifying or replacing the unmodified fan to provide amodified fan is such as to provide instead a second pressure ratio tothe core flow to the core engine replacing said first pressure ratio,wherein said first pressure ratio is greater in magnitude to said secondpressure ratio. Alternatively, said unmodified fan is further configuredfor providing a first pressure ratio to a core flow to the core engine,and wherein said modifying or replacing the unmodified fan to provide amodified fan is such as to provide instead a second pressure ratio tothe core flow to the core engine replacing said first pressure ratio,wherein said second pressure ratio is 1.0.

For example, said modified fan and said low pressure turbine areprovided in a low pressure spool assembly.

For example, additionally or alternatively to the above, the turbofanengine is configured for being operatively coupled to an electricalgenerator to enable conversion of said excess shaft power to electricalpower. Additionally or alternatively, the turbofan engine is configuredfor being operatively coupling the converted turbofan engine to amechanical load to apply said excess shaft power to the mechanical load

For example, additionally or alternatively to the above, said turbofanengine is a multi-spool, high bypass, forward fan, turbofan gas turbineengine, wherein the unmodified fan is forward mounted. For example, theturbofan engine is selectively coupled to the electrical generator viathe modified fan. Optionally, the turbofan engine is further selectivelycoupled to an additional electrical generator via the low pressureturbine. Alternatively, the turbofan engine is selectively coupled tothe electrical generator via the low pressure turbine.

For example, additionally or alternatively to the above, said turbofanengine is a multi-spool, high bypass, aft fan, turbofan gas turbineengine, wherein the unmodified fan is aft-mounted. For example, theunmodified fan is configured for providing only a bypass airflow, i.e.,does not provide a core flow to the core engine. For example, saidmodified fan and said low pressure turbine are provided in a singlerotor assembly. For example, the turbofan engine is selectively coupledto the electrical generator via the low pressure turbine.

For example, said reduced bypass thrust is nominally zero.

Additionally or alternatively, the turbofan engine further comprises athrust bearing arrangement for the low pressure turbine for balancing anext axial force corresponding to said excess shaft power.

Additionally or alternatively, the turbofan engine maintains the enginepressure ratio of the unmodified turbofan engine in the turbofan engine.For example, the low pressure turbine in the turbofan engine is modifiedwith respect to the respective low pressure turbine in the unmodifiedturbofan engine to provide a flow cross-sectional flow area through thelow pressure turbine of the turbofan at an axial location thereof thatis larger than the corresponding flow cross-sectional flow area in theunmodified turbofan engine.

Additionally or alternatively, the turbofan engine, further comprises anexhaust diffuser fitted to the core engine, wherein the exhaust diffuseris configured for reducing a core engine thrust as compared to a corethrust of the core engine in the unmodified turbofan engine.

Additionally or alternatively, said at least at one set of engineconditions includes the design point of the turbofan engine.

Additionally or alternatively, said at least at one set of engineconditions includes wherein said at least at one set of engineconditions includes maximum continuous cruise at ISA conditions of theturbofan engine.

The method and converted turbofan engine according to at least oneexample of the presently disclosed subject matter provides at least oneof the following features as compared with the conventional approach forconverting a turbofan type gas turbine to provide an aeroderivative gasturbine:

-   -   overall development time can be significantly reduced, and/or        less testing of the respective aeroderivative engine may be        required as compared with the conventional approach;    -   the respective aeroderivative engine requires relatively few        changes to the original engine hardware (i.e., the unmodified        turbofan engine) as compared with the conventional approach;    -   less new parts need to be provided for the conversion and for        production of the respective aeroderivative engine as compared        with the conventional approach.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosedherein and to exemplify how it may be carried out in practice, exampleswill now be described, by way of non-limiting example only, withreference to the accompanying drawings, in which:

FIG. 1 schematically illustrates various steps in the method ofconverting a turbofan gas turbine engine to an aeroderivative engine forgeneration of electrical power, according to at least some examples ofthe presently disclosed subject matter.

FIG. 2 schematically illustrates, in cross-sectional side view, a firstexample of an unmodified turbofan gas turbine engine.

FIG. 3 schematically illustrates, in cross-sectional side view, a firstexample of a modified turbofan gas turbine engine, converted from theunmodified turbofan gas turbine engine example of FIG. 2, based on afirst example of a modified fan.

FIG. 4 schematically illustrates, in cross-sectional side view, a firstexample of a modified turbofan gas turbine engine, converted from theunmodified turbofan gas turbine engine example of FIG. 2, based on asecond example of a modified fan.

FIG. 5 schematically illustrates, in cross-sectional side view, a firstexample of a modified turbofan gas turbine engine, converted from theunmodified turbofan gas turbine engine example of FIG. 2, based on athird example of a modified fan.

FIG. 6 schematically illustrates, in cross-sectional side view, analternative variation of the example of FIG. 3, including an example ofa thrust balancing arrangement.

FIG. 7 schematically illustrates, in cross-sectional side view, a secondexample of an unmodified turbofan gas turbine engine.

FIG. 8 schematically illustrates, in cross-sectional side view, a secondexample of a modified turbofan gas turbine engine, converted from theunmodified turbofan gas turbine engine example of FIG. 7.

DETAILED DESCRIPTION

Referring to FIG. 1, a first example of a method for converting aturbofan engine into an aeroderivative for the generation of electricity(the method being generally designated with the reference numeral 900)comprises the following steps:

-   -   Step 910˜providing a turbofan engine.    -   Step 920˜providing a converted turbofan engine from the turbofan        engine.    -   Step 930˜operatively coupling the converted turbofan engine to        an electrical generator.

Herein, the term “modified turbofan engine” is used interchangeably with“aeroderivative engine”, “converted turbofan engine”, and the like.

As will become clearer below, the turbofan engine of Step 910 comprisesat least the following components:

-   -   a core engine including at least one high pressure spool        assembly and a combustion chamber;    -   an unmodified fan configured for providing at least a bypass        flow bypassing the core engine, the fan being connected to a low        pressure turbine that is in turn driven by the core engine.

Referring also to FIG. 2, a first example of a turbofan engine,generally designated with reference numeral 100, provided in Step 910,is a multi-spool, high bypass, forward fan, turbofan gas turbine engine.

In this example, the turbofan engine 100 is an aeronautical gas turbineengine, for example a CFM56 turbofan engine, built by CFM International,which is a joint venture between GE Aviation, a division of GeneralElectric (USA) and Snecma, a division of Safran (France). However, inalternative variations of this example, or in other alternativeexamples, the turbofan engine provided in Step 910 can be any suitableturbofan engine, mutatis mutandis, for example other members of CFM 56family of turbofan engines, or other suitable turbofans.

Turbofan engine 100 comprises a core engine 110, including casing 130, ahigh pressure spool assembly 120, and combustion chamber 140. The highpressure spool assembly 120 is rotatable about longitudinal axis 150,and comprises a high pressure compressor 124 and a high pressure turbine126, axially spaced from one another by an outer shaft 125.

The combustion chamber 140 is disposed between the high pressurecompressor 124 and the high pressure turbine 126. The combustion chamber140 is, in this example an annular combustion chamber, and isoperatively connected to a fuel system 148.

Turbofan engine 100 also comprises a fan 164 mechanically coupled to alow pressure turbine 166 for rotation therewith. In this example, thefan 164 is located at a forward portion of the turbofan engine 100,forward of the core engine 110, and this mechanical coupling is viainner shaft 165. Inner shaft 165 is in coaxial relationship with outershaft 125 and turns independently thereof about longitudinal axis 150.Thus, in this example, a low pressure spool assembly 160 includes fan164, low pressure turbine 166 and inner shaft 165.

Further in this example, the low pressure spool assembly 160 alsocomprises a low pressure compressor 162, downstream of fan 164 andaccommodated in the casing 130.

In alternative variations of this example, the core engine 110 cancomprise additional spool assemblies. For example an intermediatepressure spool assembly can be provided, intermediate between the lowpressure spool assembly 160 and the high pressure spool assembly 120.

An annular bypass duct 170 is provided, having an inner wall 172 definedby an outer part of a forward portion of the casing 130, and having anouter wall 174 defined by an inner part of bypass casing 135. Bypasscasing 135 is in radially spaced relationship with respect to casing 130via radial struts 138.

The low pressure spool assembly 160 and the high pressure spool assembly120 are directly or indirectly rotatably mounted to a forward support inthe form of fan frame 137 and aft support in the form of turbine frame136, via suitable bearings.

Fan 164 is accommodated in the bypass casing 135, forward of the casing130. Fan 164 comprises a plurality of fan blades 180 radially projectingfrom fan disc 182.

Accessory drive section 145 can be provided on the outside of the bypasscasing 135. Accessory drive section 145 can include one or more of thefollowing, for example: a starter/generator; accessory gearbox; transfergearbox; horizontal driveshaft; radial driveshaft; inlet gearbox.

In conventional operation of the turbofan engine 100, the fan induces aflow W of air into the turbofan engine 100, and this flow is split atthe leading edge 131 of casing 130 into an inner airflow WI that ischanneled to the core engine 110, and a bypass airflow WO that ischanneled to the bypass duct 170.

In this example, the bypass ratio, defined as the mass flow rate ratiobetween the bypass airflow WO and inner airflow WI, is 5:1, or in theorder of 5:1, for example 4.75:1. However, in the above or otheralternative variations of this example, or in the above or otheralternative examples, the respective bypass ratio can be less than 5:1or greater than 5:1.

The inner airflow WI is compressed by the low pressure compressor 162and the high pressure compressor 124, and fed to the combustion chamber140, wherein fuel, provided by the fuel system 148, is also fed andburnt, generating hot exhaust gases. These exhaust gasses pass throughand drive the high pressure turbine 126 and then the low pressureturbine 166. In aeronautical applications, a nozzle (not shown) isprovided aft of the low pressure turbine 166 and casing 130, foraccelerating the hot exhaust gases and providing a core thrust.

The bypass airflow WO flows through the bypass duct 170, bypassing thecore engine 110, and providing in aeronautical applications a fan bypassthrust.

Thus, the core engine 110 is designed to generate hot exhaust gasescapable of driving the high pressure turbine 126, the low pressureturbine 166, and having sufficient energy to provide the required corethrust. In turn, the high pressure turbine 126 is designed to generatesufficient power to drive the high pressure compressor 124, while thelow pressure turbine 166 is designed to generate sufficient power todrive the low pressure compressor 162 and fan 164, and thus to providethe required fan bypass thrust.

As will become clearer below, Step 920 comprises providing a convertedturbofan engine from the (unmodified) turbofan engine 100, by modifyingor replacing the original unmodified fan to provide a modified fan thatis now configured for reducing bypass fan airflow generated by the fanduring operation of the engine (as compared with the bypass fan airflowgenerated with the unmodified fan in the original turbofan engine), toallow the low pressure turbine to generate an excess shaft power above abaseline shaft power that is required for driving said modified fanduring operation of the converted turbofan engine.

Referring also to FIG. 3, the turbofan engine 100 is converted to amodified or converted turbofan 200, comprising, i.e., retaining, thecore engine 110, including the high pressure spool assembly 120 and thecombustion chamber 140, and also comprising, i.e., retaining, the lowpressure turbine 166. As is clear from the presently disclosed subjectmatter, the low pressure turbine 166 is initially designed to drive theunmodified fan 164, the unmodified fan 164 being configured forproviding at least a bypass airflow bypassing the core engine togenerate a fan bypass thrust when driven by said low pressure turbine.(In this example the unmodified fan 164 is also configured forproviding, a core airflow through the core engine 110 for operationthereof.)

However, in the converted or modified turbofan engine, the original orunmodified fan 164 is modified to provide, or alternatively is replacedby, a modified fan, which is coupled to the low pressure turbine 166 viathe correspondingly modified low pressure spool assembly 260, in asimilar manner to the coupling of the unmodified fan 164 to the lowpressure turbine 166 via the unmodified low pressure spool assembly 160as disclosed herein, mutatis mutandis. Similarly, in the modifiedturbofan engine 200, the low pressure turbine 166 is driven by hot gasesgenerated by the core engine 110, thereby turning the modified fan.

The modified fan is configured for generating a reduced bypass airflow(typically accompanied by a reduced fan thrust) during operation of themodified bypass engine 200, as compared with the bypass fan airflow (andbypass thrust, respectively), generated by the unmodified fan (atsimilar engine operating conditions). Thereby, the low pressure turbine166 generates an excess shaft power above a baseline shaft power that isrequired only for driving said modified fan during operation of theconverted turbofan engine. The greater the reduction in the bypassairflow WO (and thus typically of bypass fan thrust), the greater is theexcess shaft power generated by the low pressure turbine 166, and thisexcess shaft power can be maximized by reducing the bypass fan thrust orbypass airflow to zero, or near thereto.

In other words, by removing or substantially reducing the requirementfor the modified fan to generate bypass airflow, the shaft power thatwas formerly required for driving the unmodified fan to generate such abypass airflow (and typically bypass fan thrust) in the originalturbofan engine 100 now becomes available shaft power, and thus forconversion subsequently to electrical power.

The modified fan is thus configured to provide a significantly reducedbypass airflow WOR, up to a zero bypass airflow, as compared to thebypass airflow WO generated by the unmodified turbofan engine 100 withthe unmodified fan 164, at similar operating conditions (i.e., atsimilar engine conditions).

In practice it can be beneficial to retain a small proportion of thebypass mass flow, to improve the aerodynamic quality of the flowentering into the core engine and/or this minimal bypass flow canprovide cooling benefits to the core engine. An optimal condition can befound in which engine performance of the core engine can be maximized onthe one hand and the bypass flow minimized on the other hand.

Concurrently, the unmodified fan 164 is configured for providing a firstpressure ratio to a core flow that flows to the core engine; modifyingor replacing the unmodified fan to provide the modified fan is such asto provide instead a second pressure ratio to the core flow that flowsto the core engine (thereby replacing the first pressure ratio), whereinthe first pressure ratio is similar in magnitude to the second pressureratio. However, it is possible for the first pressure ratio to be largerin magnitude to the second pressure ratio, i.e., modifying or replacingthe unmodified fan to provide a modified fan can result in the secondpressure ratio to be smaller than the first pressure ratio. On the otherhand, it is also possible to provide a second pressure ratio that is infact larger than the first pressure ratio, for example by increasing therpm of the low pressure spool.

Referring again to FIG. 3, in a first example of the modified fan,designated herein by the reference numeral 164A, the modified fan 164Acomprises modified fan blades 180A, having root to tip spans SA that aresignificantly shorter than the respective spans S of the unmodified fanblades 180 of the unmodified turbofan engine 100. In this example, theoriginal fan blades 180 are worked on to mechanically remove a radiallyouter portion 181 of each of the fan blades 180, so that only a radiallyinner portion 183 of the fan blades 180 remain in the modified fanblades 180A. Thus, the outer diameter DA of modified fan 164A issignificantly smaller than the corresponding outer diameter D of theunmodified fan 164. Optionally, the modified fan 164A can be furthermodified by providing an annular shroud (not shown) connecting the tipsT of the modified fan blades 180A.

Alternatively rather than modifying the existing fan blades 180, thesecan be removed and replaced with new modified fan blades 180A, or thewhole fan 180 can be replaced with a new fan having modified fan blades180A with reduced diameter viz-a-viz the unmodified fan blades.

In this example, the tips T of the modified fan blades 180A extendradially to the radial position of the leading edge 131 of casing 130,or just beyond—for example, the radial position of the edge 131 can beoptimized to provide optimized performance for the modified engine inwhich engine performance of the core engine can be maximized on the onehand and the bypass flow minimized on the other hand, thereby providinga small bypass flow which can be beneficial, as mentioned above. Bydoing so, the core airflow WI can remain relatively unaffected by theabove modification of the fan, thereby ensuring that the compressionpressure ratio of the core airflow into the core engine 110 remainsessentially the same as in the unmodified turbofan engine 100 (for thesame set of operating (engine) conditions). In alternative variations ofthis example, the tips T of the modified fan blades 180A do not extendradially to the radial position of the leading edge 131 of casing 130,but instead are located at a radial position inwards of that of theleading edge 131.

In at least this example (and optionally also for the second and thirdexamples below and alternative variations thereof, mutatis mutandis),the bypass casing 135 is retained, and the accessories 145 can also beretained, to minimize additional modifications to the modified turbofanengine 200. However, in alternative variations of these examples and inother examples, it is also possible to remove the bypass casing 135and/or the accessories 145.

In this example, the combustion chamber 140 is unchanged with respect tothe unmodified turbofan gas turbine 100, and thus continues to workbased on liquid fuel. However, on alternative variations of this or ofother examples, or in other examples, the combustion chamber can bemodified, or replaced, to enable combustion of gas fuels, or indeed ofsolid fuels.

Referring to FIG. 4, in a second example of the modified fan, designatedherein by the reference numeral 164B, replaces the modified fan 164A ofthe modified engine 200 of FIG. 3, mutatis mutandis.

The modified fan 164B has the fan blades 180 removed, optionallyretaining the fan disc 182. Further optionally, plugs 180B can beprovided to cover the openings left in the periphery of the fan disc182, for example such openings corresponding to the fir tree root designof conventional fan blades. In this example, the absence of the fanblades 180 provides a slightly reduced core airflow WIR as compared withthe core airflow WI of the unmodified turbofan engine 100, andcorrespondingly, the compression pressure ratio of the modified turbofanengine 200 is correspondingly less than that of the unmodified turbofanengine 100 (for the same set of engine operating conditions), loweringthe available excess shaft power generated by the respective modifiedturbofan engine 200.

Thus, the unmodified fan 164 is configured for providing a firstpressure ratio to a core flow that flows to the core engine; modifyingor replacing the unmodified fan to provide modified fan 164B is such asto provide instead a second pressure ratio to the core flow to the coreengine (thereby replacing the first pressure ratio), wherein the saidsecond pressure ratio is effectively 1.0.

Referring to FIG. 5, in a third example of the modified fan, designatedherein by the reference numeral 164C, replaces the modified fan 164A ofthe modified engine 200 of FIG. 3, mutatis mutandis.

The modified fan 164C comprises modified fan blades 180C, in which anouter radial portion 181C of the fan blades 180C generates reducedthrust and reduced bypass airflow, as compared with the unmodified fanblades 180 of the unmodified turbofan engine 100. Referring also to FIG.5(a), outer radial portion 181C of each fan blade 180C has a zero ornear zero angle of attack with respect to a direction of the airflowinto the converted turbofan engine 200. The inner radial portion 183C ofeach blade 180C has a geometry similar to that of the correspondingportion of the unmodified fan blades 180, having the appropriatecorresponding angle of attack. In this example, the original fan blades180 are worked on to mechanically twist the radially outer portion 181Cof the fan blades 180 in the appropriate direction, to provide therequired modified blade geometry of fan blades 180C. Thus, the radialportion 181C of each fan blade 180C of the modified fan 164C does notprovide a significant bypass airflow or bypass thrust. Alternativelyrather than modifying the existing fan blades 180, these can be removedand replaced with new modified fan blades 180C, or the whole fan 180 canbe replaced with a new fan having modified fan blades 180C.

In this example, the radial position of the transition T′ between theouter radial portion 181C and the inner radial portion 183C of themodified fan blades 180C corresponds to the radial position of theleading edge 131 of casing 130, or just beyond. In this manner, the coreairflow WI can remain relatively unaffected by the above modification ofthe fan, thereby ensuring that the compression pressure ratio of thecore airflow into the core engine 110 remains essentially the same as inthe unmodified turbofan engine 100 (for the same set of engineconditions, i.e., for the same set of operating conditions). Inalternative variations of this example, the radial position of thetransition T′ of the modified fan blades 180C do not extend radially tothe radial position of the leading edge 131 of casing 130, but insteadare located at a radial position inwards of that of the leading edge131, including a position up to the respective fan blade roots.

It is to be noted that the aforementioned reduction in bypass flowtypically provides also a reduction of bypass thrust, which can resultin a net axial force being applied to low pressure spool assembly 160 inan aft direction. In some cases, the low pressure spool assembly 160 isalready configured for operating under such circumstances; in othercases, it can be possible to modify the bearings of the low pressurespool assembly 160 to withstand the new loads arising from the net axialforce being applied to low pressure spool assembly 160. In a variationof the example in FIG. 3, and in at least some other examples (forexample, the examples illustrated in FIGS. 4 and 5, mutatis mutandis),the modified turbofan gas turbine 200 (including the modified fanaccording to any one of the first to third examples herein oralternative variations thereof or other examples thereof according tothe presently disclosed subject matter) can be further modified byproviding a suitable thrust balancing arrangement for providing thrustload compensation to the bearings of the low pressure spool assembly160. For example, and referring to FIG. 6, such a thrust balancingarrangement can comprise a balance piston 300, comprising a piston head310 fixed on the output shaft 330 to turn therewith with respect to apiston casing 312. The piston head 310 is thus mechanically connected tothe inner shaft 165. One side of the piston is exposed to high pressureair, provided via a bleed line 314 in fluid communication with the highpressure compressor 120, while the other side of the piston head 310 isexposed to a different pressure, for example a lower pressure such asfor example ambient air, to provide a pressure difference ΔP across thepiston head. This arrangement results in providing a forwardcompensating force to the low pressure spool via the inner shaft 165,and by controlling the pressure difference ΔP across the piston head andthe area over which this pressure difference acts, the resultingcompensating force can be provided that balances the aforesaid net axialforce, for example similar to the original force generated by theunmodified fan. Alternatively, the compensating force to the net axialforce can be provided via a hydraulic piston arrangement, or via amechanical piston arrangement, or any other suitable force compensatingarrangement.

Referring again to FIGS. 3, 4 and 5, for example, a diffuser 190 canoptionally be provided aft of the low pressure turbine 166 to minimizethe core thrust, or to reduce the core thrust close to zero. By doingso, more of the energy available in the hot gases can be extracted bythe low power turbine 166, thereby increasing further the aforementionedexcess shaft power.

Typically, the original engine pressure ratio of the turbofan engine ismaintained in the modified turbofan engine. This can be useful inallowing the modified turbofan engine to operate in an optimum mannersimilar to the unmodified turbofan engine. In at least some examples,including one or more of the above examples, the original enginepressure ratio of the turbofan engine is maintained in the modifiedturbofan engine by modifying the low pressure turbine in the modifiedturbofan engine to provide a flow cross-sectional flow area through thelow pressure turbine at an axial location thereof that is larger thanthe corresponding flow cross-sectional flow area in the unmodifiedturbofan engine. For example, the last stage of stators of the lowpressure turbine can be modified to increase the flow area therethrough,for example by changing the pitch of the stators.

A man of ordinary skill in the art appreciates that the modifiedturbofan engine 200 needs to be suitably controlled to provide thedesired performance, and some further adjustments of the modified engine200 may also be necessary. For example, a series of tests and/ornumerical simulations may be required to determine optimum operatingconditions (i.e. optimum engine conditions), for example matching thehigh pressure spool with respect to the low pressure spool.

As will become clearer below, Step 930 comprises operatively connectingthe converted turbofan engine to an electrical generator to enableconversion of said excess shaft power to electrical power. It is to benoted, though, that the excess shaft power can be used in a differentmanner, for example for driving a mechanical load.

Referring again to FIGS. 3, 4 and 5, an engine power output shaft 330 iscoupled to, and is driven by, the low pressure spool 160. In thisexample, the engine power output shaft 330 is coupled to the lowpressure spool 160 at the front end of the engine 200, thereby avoidinginteraction or interference with the hot exhaust gases. In particular,the engine power output shaft 330 is coupled to the respective modifiedfan 164A, 164B, 164C, either directly or via a universal joint, orexample. However, in alternative variations of this example, the enginepower output shaft 330 can be coupled, instead, to the low pressurespool 160 at the aft end of the engine 200, for example by directcoupling to the low pressure turbine 166. In yet other alternativevariations of this example, an additional engine power output shaft canbe coupled to the low pressure spool 160 at the aft end of the engine200, for example by direct coupling to the low pressure turbine 166, sothat the modified turbofan engine 200 is coupled to two electricalgenerators, one via the low pressure turbine, and the other via themodified fan.

In turn, the engine power output shaft 330 is also coupled to a gearbox310, while an output shaft 315 of the gearbox 310 is connected to theinput shaft 316 of electrical generator 320. The electric generator 320can include any one of a number of suitable electrical generators thatgenerate electrical power, responsive to the input shaft 316 beingmechanically turned. For example, the electrical generator can be of thepermanent magnet type, and can be configured for providing single phase,or three-phase power at any desired voltage. Examples of suitableelectrical generators can include: Sgen-100A 4p provided by Siemens(Germany); 6A8 Series provided by GE (USA). Examples of suitablegearboxes can include high speed single stage parallel shaft gearboxes,provided by Flender Graffenstaden (EU) or by Elecon (India).

Thus, the converted turbofan engine 200 is configured for beingoperatively connected to an electrical generator 320 to enableconversion of said excess shaft power to electrical power.

In at least one of the above examples in which the converted turbofanengine 200 is converted from a CFM 56-3 turbofan engine, it iscontemplated that such a converted turbofan engine 200 can generate forexample about 15 MW of electrical power.

For example, in theoretical studies conducted by the applicants, atcontinuous ISA conditions, the unmodified CFM 56-3 turbofan enginegenerates a net thrust of 21,995 lbf, with a thrust specific fuelconsumption of 0.39. At the same conditions, including a fuel flow rateof 8686 lbm/hr, engine pressure ratio of 3.99, the modified turbofanengine (modified as per the example illustrated in FIG. 3) generates ashaft power of 15,551 kW, with shaft efficiency of 32.9%, a shaft heatrate of 10372 Btu/kWh and shaft efficiency of 0.42 lbm/hp·hr(concurrently, the modified turbofan engine generates a net thrust of3530 lbf).

Referring to FIG. 7, a second example of a turbofan engine, generallydesignated with reference numeral 1100, provided in Step 910, is amulti-spool, high bypass, aft fan, turbofan gas turbine engine. Turbofanengine 1100 is similar to turbofan engine 100, mutatis mutandis, butwith some differences, as follows.

Turbofan engine 1100 comprises a core engine 1110, including casing1130, a high pressure spool assembly 1120 (rotatable about longitudinalaxis 1150, and comprising a high pressure compressor 1124 and a highpressure turbine 1126, axially spaced from one another by an outer shaft1125), and combustion chamber 1140, similar to the correspondingcomponents of turbofan engine 100, mutatis mutandis: core engine 110,casing 130, high pressure spool assembly 120 (longitudinal axis 150,high pressure compressor 124, high pressure turbine 126, outer shaft125), and combustion chamber 140.

Turbofan engine 1100 also comprises a fan 1164 mechanically coupled to alow pressure turbine 1166 for rotation therewith. In this example, thefan 1164 is located at a aft portion of the turbofan engine 1100, aft ofthe core engine 1110, and this mechanical coupling is provided by havingthe fan 1164 and the low pressure turbine 1166 in a single rotorassembly 1160.

An annular bypass duct 1170 is provided, having an inner wall 1172defined by an outer part of an aft casing 1133 that is joined to thecasing 1130, aft of the high pressure turbine 1126, and having an outerwall 1174 defined by an inner part of bypass casing 1135. Bypass casing1135 is in radially spaced relationship with respect to aft casing 1133via radial struts 1138.

The rotor assembly 1160 comprises a rotor disc 1182, and a plurality ofdual aerofoils 1190, mounted on the periphery of the rotor disc 1182 incircumferential spaced relationship, and radially projecting from therotor disc 1182.

Each dual aerofoil 1190 comprises an inner turbine blade portion 1192joined to an outer fan blade portion 1194 via platform 1193, and in thisexample, each dual aerofoil 1190 is formed as an integral structure. Therespective inner turbine blade portions 1192 form the low pressureturbine 166, while the outer fan blade portions 194 form the fan 1166.The platforms 1193 join one another to form an annular ring thatrotatably seals with respect to the aft casing 1133. Thus, the fan 1164is accommodated in the bypass casing 1135, while the low pressureturbine 1164 is accommodated in the aft casing 1133.

The rotor assembly 1160 is in coaxial relationship with shaft 1125 andturns independently thereof about longitudinal axis 1150.

A nozzle 1191 is provided aft of aft casing 1133.

In conventional operation of the turbofan engine 1100, an inner airflowWI′ is induced into the core engine 1110, and a bypass airflow WO′ isinduced into the bypass duct 1170.

In this example, the bypass ratio, defined as the mass flow rate ratiobetween the bypass airflow WO′ and inner airflow WI′, is in the order of5:1, for example 5:1. However, in the above or other alternativevariations of this example, or in the above or other alternativeexamples, the respective bypass ratio can be less than 5:1 or greaterthan 5:1.

The inner airflow WI′ is compressed by the high pressure compressor1124, and fed to the combustion chamber 1140, wherein fuel, provided bythe fuel system (not shown), is also fed and burnt, generating hotexhaust gases. These exhaust gasses pass through and drive the highpressure turbine 1126 and then the low pressure turbine 1166. Inaeronautical applications, nozzle 1191 accelerates the hot exhaust gasesand providing a core thrust.

The bypass airflow WO′ flows exclusively through the bypass duct 1170,completely bypassing the core engine 1110, and providing in aeronauticalapplications a fan bypass thrust.

Thus, the core engine 1110 is designed to generate hot exhaust gasescapable of driving the high pressure turbine 1126, the low pressureturbine 1166, and having sufficient energy to provide the required corethrust. In turn, the high pressure turbine 1126 is designed to generatesufficient power to drive the high pressure compressor 1124, while thelow pressure turbine 1166 is designed to generate sufficient power todrive the low pressure compressor fan 1164, and thus to provide therequired fan bypass thrust.

As will become clearer below, in the second example Step 920 comprisesproviding a converted turbofan engine from the turbofan engine 1100, bymodifying or replacing the original unmodified fan 1164 to provide amodified fan that is now configured for reducing bypass fanthrust/bypass airflow generated by the fan during operation of theengine (as compared with the bypass fan thrust/bypass airflow generatedwith the unmodified fan in the original turbofan engine), to allow thelow pressure turbine to generate an excess shaft power above a baselineshaft power that is required for driving said modified fan duringoperation of the converted turbofan engine.

Referring also to FIG. 8, the turbofan engine 1100 is converted to amodified or converted turbofan 1200, comprising, i.e., retaining, thecore engine 1110, including the high pressure spool assembly 1120 andthe combustion chamber 1140, and also comprising, i.e., retaining, thelow pressure turbine 1166. As is clear from the presently disclosedsubject matter, the low pressure turbine 1166 is initially designed todrive the unmodified fan 1164, the unmodified fan 1164 being configuredfor providing at least a bypass airflow bypassing the core engine togenerate a fan bypass thrust when driven by said low pressure turbine.(In this example the unmodified fan 1164 is also configured forproviding only the bypass flow and does not contribute to the coreairflow through the core engine 1110.)

However, in this example of the converted or modified turbofan engine1200, the original or unmodified fan 1164 is essentially removed byremoving part or all of each of the outer fan blade portion 1194 of thedual aerofoils 1190, preferably up to but not including the respectiveplatforms 1193, though in alternative variations of this example part ofthe respective outer fan blade portion 1194 can be retained. The resultis to have the original or unmodified fan 1164 modified to, or replacedby, what is conveniently referred to herein as a “modified fan”, anddesignated with the reference numeral 1166A, even though such a“modified fan” can have no fan blades at all, or very short fan blades.Such a “modified fan” 1166A, which in this example can comprise only theouter parts of the platforms 1193 when all the fan blade portions 1194are removed, is coupled to the low pressure turbine 1166 via thecorrespondingly modified single rotor assembly 1260, in a similar mannerto the coupling of the unmodified fan 1164 to the low pressure turbine1166 via the unmodified rotor assembly 1160 as disclosed herein, mutatismutandis.

Similarly, in the modified turbofan engine 1200, the low pressureturbine 1166 is driven by hot gases generated by the core engine 1110,thereby turning the modified fan 1166A.

The modified fan is configured for generating a reduced or zero bypassfan thrust and/or modified bypass airflow during operation of themodified bypass engine 1200, as compared with the bypass fan thrustand/or bypass airflow, respectively, generated by the unmodified fan (atsimilar engine operating conditions). Thereby, the low pressure turbine1166 generates an excess shaft power above a baseline shaft power thatis required only for driving said modified fan during operation of theconverted turbofan engine 200. In the above example where no bypassairflow is generated, the baseline shaft power is at a minimum, and theexcess shaft power can be maximized.

It is to be noted that the aforementioned excess shaft power can resultin a net axial force being applied to rotor assembly 1160. Accordingly,the modified turbofan gas turbine 1200 can be further modified byproviding a suitable arrangement for providing thrust load compensationto the bearings of the rotor assembly 1160, for example a balance pistonas disclosed above, mutatis mutandis.

Referring again to FIG. 8, a diffuser 1197 can be provided aft of thelow pressure turbine 1166 to replace the nozzle 1191, and thus minimizethe core thrust, or reduce the core thrust to zero. By doing so, more ofthe energy available in the hot gases can be extracted by the low powerturbine 1166, thereby increasing further the aforementioned excess shaftpower.

In this example, Step 930 comprises operatively connecting the convertedturbofan engine to an electrical generator to enable conversion of saidexcess shaft power to electrical power. It is to be noted, though, thatthe excess shaft power can be used in a different manner, for examplefor driving a mechanical load.

Referring again to FIG. 8, an engine power output shaft 1330 is coupledto, and is driven by, the rotor assembly 1160. In this example, theengine power output shaft 1330 is coupled to the low pressure spool 160at the aft end of the engine 1200, and a suitable diverter duct 1199 canbe provided aft of the diffuser 1197 to divert the hot gases away fromthe aft end of the output shaft 1330. In particular, the engine poweroutput shaft 1330 is coupled to the rotor assembly 1160, either directlyor via a universal joint, or example. In turn, the engine power outputshaft 1330 is also coupled to a gearbox 1310, while an output shaft 1315of the gearbox 1310 is connected to the input shaft 1316 of electricalgenerator 1320. The electric generator 1320 can include any one of anumber of suitable electrical generators that generate electrical power,responsive to the input shaft 1316 being mechanically turned, forexample as disclosed above for the first example, mutatis mutandis.

Thus, the converted turbofan engine 1200 is configured for beingoperatively connected to an electrical generator 1320 to enableconversion of said excess shaft power to electrical power.

In the method claims that follow, alphanumeric characters and Romannumerals used to designate claim steps are provided for convenience onlyand do not imply any particular order of performing the steps.

Finally, it should be noted that the word “comprising” as usedthroughout the appended claims is to be interpreted to mean “includingbut not limited to”.

While there has been shown and disclosed examples in accordance with thepresently disclosed subject matter, it will be appreciated that manychanges may be made therein without departing from the spirit of thepresently disclosed subject matter.

1. A method for converting a turbofan engine, comprising; (a) providing a turbofan engine, the turbofan engine comprising; a core engine including at least one high pressure spool assembly and a combustion chamber; an unmodified fan configured for providing at least a bypass flow bypassing the core engine at least at one set of engine conditions, the fan being mechanically coupled to a low pressure turbine that is in turn driven by the core engine; (b) providing a converted turbofan engine from the turbofan engine by modifying or replacing the unmodified fan to provide a modified fan, the modified fan being configured for generating a reduced bypass flow with respect to said fan bypass flow during operation of the converted turbofan engine corresponding to said at least one set of engine conditions of step (a), to thereby enable said low pressure turbine to generate an excess shaft power above a baseline shaft power required for driving said modified fan during operation of the converted turbofan engine.
 2. The method according to claim 1, wherein in step (b), said modified fan is modified with respect to the unmodified fan by reducing the outer diameter of the fan blades of said unmodified fan.
 3. The method according to claim 1 or claim 2, wherein in step (b), said modified fan is modified with respect to the unmodified fan by removing at least an outer radial portion of the fan blades of said unmodified fan.
 4. The method according to any of claims 1 to 3, wherein in step (b), said modified fan is modified with respect to the unmodified fan by removing the fan blades of said unmodified fan.
 5. The method according to claim 1, wherein in step (b), said modified fan is modified with respect to the unmodified fan by modifying the geometry of at least an outer radial portion of the fan blades of said unmodified fan such that the said at least outer radial portion of the fan blades generates reduced bypass flow as compared with the unmodified fan blades.
 6. The method according to any one of claims 1 to 5, wherein in step (a), the unmodified fan is also configured for providing a core flow through the core engine.
 7. The method according to claim 6, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said first pressure ratio is similar in magnitude to said second pressure ratio.
 8. The method according to claim 6, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said first pressure ratio is smaller in magnitude to said second pressure ratio.
 9. The method according to claim 6, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said first pressure ratio is larger in magnitude to said second pressure ratio.
 10. The method according to claim 6, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine, wherein said second pressure ratio is 1.0.
 11. The method according to any one of claims 1 to 10, wherein said modified fan and said low pressure turbine are provided in a low pressure spool assembly.
 12. The method according to any one of claims 1 to 11, further comprising operatively coupling the converted turbofan engine to an electrical generator to enable conversion of said excess shaft power to electrical power.
 13. The method according to any one of claims 1 to 12, wherein said turbofan engine in step (a) is a multi-spool, high bypass, forward fan, turbofan gas turbine engine, wherein said unmodified fan is forward mounted.
 14. The method according to any one of claims 12 to 13, wherein the converted turbofan engine is selectively coupled to the electrical generator via the modified fan.
 15. The method according to any one of claims 12 to 14, wherein the converted turbofan engine is coupled to the electrical generator via the low pressure turbine.
 16. The method according to any one of claims 1 to 12, wherein said turbofan engine in step (a) is a multi-spool, high bypass, aft fan, turbofan gas turbine engine, wherein said unmodified fan is aft-mounted.
 17. The method according to any one of claim 1 to 5, 11, 12 or 16, wherein in step (a), the unmodified fan is configured for providing only a bypass airflow.
 18. The method according to claim 17, wherein said modified fan and said low pressure turbine are provided in a single rotor assembly.
 19. The method according to any one of claim 14, 17 or 18, wherein the converted turbofan engine is coupled to the electrical generator via the low pressure turbine.
 20. The method according to any one of claims 1 to 5, 11, 12 or 16 to 19, wherein said reduced bypass thrust is nominally zero.
 21. The method according to any one of claims 1 to 20, further comprising providing a thrust bearing arrangement for the low pressure turbine for balancing a next axial force corresponding to said excess shaft power.
 22. The method according to any one of claims 1 to 21, further comprising maintaining the engine pressure ratio of the turbofan engine in the modified turbofan engine.
 23. The method according to claim 22, comprising modifying the low pressure turbine in the modified turbofan engine to provide a flow cross-sectional flow area through the low pressure turbine at an axial location thereof that is larger than the corresponding flow cross-sectional flow area in the turbofan engine.
 24. The method according to any one of claims 1 to 23, further comprising replacing a core nozzle of the unmodified engine of step (a) with an exhaust diffuser to the core engine in step (b), wherein the exhaust diffuser is configured for reducing a core engine thrust as compared to a core thrust of the core engine in the unmodified turbofan engine.
 25. The method according to any one of claims 1 to 24, wherein said at least at one set of engine conditions includes at least one of the design point of the turbofan engine and the maximum continuous cruise at ISA conditions of the turbofan engine.
 26. A turbofan engine, comprising; a core engine including at least one high pressure spool assembly and a combustion chamber; a modified fan mechanically coupled to a low pressure turbine that is in turn driven by hot gases generated by the core engine; wherein said modified fan is derived from or replaces an unmodified fan; wherein said low pressure turbine is initially designed to drive the unmodified fan, when coupled thereto in place of the modified fan, the unmodified fan being configured for providing at least a bypass flow bypassing the core engine at least at one set of engine conditions when driven by said low pressure turbine; wherein said modified fan is configured for generating a reduced bypass flow with respect to said fan bypass flow during operation of the turbofan engine corresponding to said at least one set of engine conditions, and for concurrently allowing said low pressure turbine to generate an excess shaft power above a baseline power required for driving said modified fan during operation of the turbofan engine at least at said at least one set of engine conditions.
 27. The turbofan engine according to claim 26, wherein said turbofan engine is produced by converting an unmodified turbofan engine, the unmodified turbofan engine comprising said core engine and said unmodified fan, said unmodified fan being mechanically coupled to the low pressure turbine that is in turn driven by hot gases generated by the core engine, and wherein said conversion includes modifying or replacing the unmodified fan to provide the modified fan.
 28. The turbofan engine according to claim 26 or claim 27, wherein said modified fan is configured to produce a reduced bypass thrust, as compared with the bypass thrust generated by the unmodified fan when coupled to the turbofan engine in place of the modified fan.
 29. The turbofan engine according to any one of claims 26 to 28, wherein said modified fan is modified with respect to the unmodified fan by reducing the outer diameter of the fan blades of said unmodified fan.
 30. The turbofan engine according to any one of claims 26 to 29, wherein said modified fan is modified with respect to the unmodified fan by removing at least an outer radial portion of the fan blades of said unmodified fan.
 31. The turbofan engine according to any one of claims 26 to 30, wherein said modified fan is modified with respect to the unmodified fan by removing the fan blades of said unmodified fan.
 32. The turbofan engine according to any one of claims 26 to 28, wherein said modified fan is modified with respect to the unmodified fan by modifying the geometry of at least an outer radial portion of the fan blades of said unmodified fan such that the said at least outer radial portion of the fan blades generates reduced bypass flow as compared with the unmodified fan blades.
 33. The turbofan engine according to any one of claims 26 to 32, wherein the unmodified fan is further configured for providing a core flow through the core engine.
 34. The turbofan engine according to claim 33, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said first pressure ratio is similar in magnitude to said second pressure ratio.
 35. The turbofan engine according to claim 33, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said first pressure ratio is smaller in magnitude to said second pressure ratio.
 36. The turbofan engine according to claim 33, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said first pressure ratio is greater in magnitude to said second pressure ratio.
 37. The turbofan engine according to claim 33, wherein said unmodified fan is further configured for providing a first pressure ratio to a core flow to the core engine, and wherein said modifying or replacing the unmodified fan to provide a modified fan is such as to provide instead a second pressure ratio to the core flow to the core engine replacing said first pressure ratio, wherein said second pressure ratio is 1.0.
 38. The turbofan engine according to any one of claims 26 to 37, wherein said modified fan and said low pressure turbine are provided in a low pressure spool assembly.
 39. The turbofan engine according to any one of claims 26 to 38, wherein the turbofan engine is configured for being operatively coupled to an electrical generator to enable conversion of said excess shaft power to electrical power.
 40. The turbofan engine according to any one of claims 26 to 39, wherein said turbofan engine is a multi-spool, high bypass, forward fan, turbofan gas turbine engine, wherein the unmodified fan is forward mounted.
 41. The turbofan engine according to any one of claims 26 to 40, wherein the turbofan engine is selectively coupled to the electrical generator via the modified fan.
 42. The turbofan engine according to any one of claims 26 to 41, wherein the turbofan engine is selectively coupled to the electrical generator via the low pressure turbine.
 43. The turbofan engine according to any one of claims 26 to 39, wherein said turbofan engine is a multi-spool, high bypass, aft fan, turbofan gas turbine engine, wherein the unmodified fan is aft-mounted.
 44. The turbofan engine according to any one of claim 26 to 39 or 43, wherein the unmodified fan is configured for providing only a bypass airflow.
 45. The turbofan engine according to any one of claims 26 to 39 or 43 to 44, wherein said modified fan and said low pressure turbine are provided in a single rotor assembly.
 46. The turbofan engine according to any one of claims 26 to 39 or 43 to 45, wherein the turbofan engine is selectively coupled to the electrical generator via the low pressure turbine.
 47. The turbofan engine according to any one of claims 26 to 46, further comprising a thrust bearing arrangement for the low pressure turbine for balancing a next axial force corresponding to said excess shaft power.
 48. The turbofan engine according to any one of claims 27 to 47, wherein the turbofan engine maintains the engine pressure ratio of the unmodified turbofan engine in the turbofan engine.
 49. The turbofan engine according to claim 48, wherein the low pressure turbine in the turbofan engine is modified with respect to the low pressure turbine in the unmodified turbofan engine to provide a flow cross-sectional flow area through the low pressure turbine of the turbofan at an axial location thereof that is larger than the corresponding flow cross-sectional flow area in the unmodified turbofan engine.
 50. The turbofan engine according to any one of claims 26 to 49, further comprising an exhaust diffuser fitted to the core engine, wherein the exhaust diffuser is configured for reducing a core engine thrust as compared to a core thrust of the core engine in the unmodified turbofan engine. 